The present invention relates to cable television (CATV) receivers. More particularly, the present disclosure relates to a method and apparatus for measuring the susceptibility to intermodulation distortion of digital CATV receivers.
Intermodulation distortion generally occurs in a digital receiver, such as a cable modem or set-top box, when the total power of all signals input to the receiver is too high relative to the power of the digital signal desired to be demodulated. However, if the relative power level of the desired digital channel could be increased, the receiver might be able to attenuate all incoming signals and substantially prevent the intermodulation distortion from occurring.
When intermodulation distortion occurs, the receiver experiences decode errors that are not caused by impairments within the digital channel. Existing test instruments can measure the bit error rate caused by impairments within a digital channel, but not those caused by intermodulation distortions generated within the receiver, such as a cable modem, set-top box, or other consumer-grade receiver. Test instrument receivers may be capable of rejecting relatively high signal amplitude levels that fall outside of the particular frequency range of the digital channel being measured. On the other hand, the receivers used in consumer-grade equipment are typically not so capable. Thus, the test instruments are unable to measure or accurately estimate bit errors caused by intermodulation distortion at a consumer-grade receiver. Therefore, such test instruments will generally fail to report problems that cause impairments in consumer-grade equipment.
Unfortunately, the difference in amplitude levels between the lowest and highest frequencies carried on a cable is typically not a constant value. A coaxial cable, for example, attenuates higher frequencies more than lower frequencies. This effect is known as tilt. In order to compensate for attenuation, amplifiers are used to boost the overall signal power. Equalizers may be used in conjunction with the amplifiers so that the net signal boost is greater for higher frequencies, thereby reducing the tilt.